Mobile communication networks are currently evolving from circuit switched (CS) networks towards packet switched (PS) networks, and by that integrate into IP based infrastructure that are e.g. used for the Internet, the World Wide Web and the datacom industry. With this trend mobile networks closely follow the evolution steps of the wireline networks, where Voice over IP (VoIP) via DSL access or via WLAN access is existing technology today.
So-called IP Multimedia Subsystem (IMS) networks have been developed for delivering multimedia services to mobile terminals (e.g. to GSM terminals) being designed according to the well-known standard named Global System for Mobile Communication (GSM), or to Wideband Code Division Multiplex Access (WCDMA) stations. Hereto, calls from and to subscribers of the multimedia services using a CS access are routed through the IMS network in order to reach an IMS service engine. This concept is called IMS Centralized Services (ICS) being described in the standardization document TS 23.292, release 8, of the so-called Third Generation Partnership Project (3GPP) that is a standardization body to produce globally applicable technical specifications.
A further 3GPP standard document in this scope is TS 23.237 (current release 10.1) specifying architectural requirements and procedures for delivery of PS-CS or CS-PS IMS Service Continuity.
In the frame of the 3GPP, further a project called Long Term evolution (LTE) has been established to enhance the UMTS mobile phone standard to cope with future requirements. Within this project, the packet core is being developed to the so-called Evolved Packet Core (EPC) forming a part of the Evolved Packet System (EPS) that supports the so-called evolved UMTS Terrestrial Radio Access Network (eUTRAN) being a new packet oriented radio access (PS access network).
As part of the 3GPP standardization, e.g. being described in the document TS 23.216 (current release 9.3), work on call continuity for terminals equipped with single radio means (i.e. terminals with one single radio transmitter and one single radio receiver thus being capable of transmitting/receiving on only one of PS- or CS-access at a given time) being referred to as Single Radio Voice Call Continuity (SR-VCC) is ongoing, enabling to transfer an IMS voice call from the EPS to the CS and vice versa.
According methods described in the above-cited document TS 23.216, SR-VCC between PS access and CS access is based on the principle that an MSC being enhanced for SR-VCC, also being referred to as evolved MSC (eMSC), initiates a session transfer towards an application server being dedicated to maintain service continuity when handing over from an actual access network to a target access network, also being referred to as Service Centralization and Continuity Application Server (SCC-AS).
To perform a fast handover from the CS network to the PS network and vice versa, the UE should preferably already be registered in the IMS network and having at least a signaling bearer in the PS network when the handover takes place.
When the UE moves from one access network to another access network, there is a possibility that the registration of the UE in the IMS network will expire because of a missing PS bearer support for an IMS re-registration. This could happen if the UE, having an ongoing CS call, enters an access network, which does not support simultaneous PS and CS bearers, e.g. Global System for Mobile communications (GSM) networks or Enhanced Radio Access Networks (GERAN) not having a Dual Transfer Mode (DTM) support.